Convert a list of characters into a string
Question:
If I have a list of chars:
a = ['a','b','c','d']
How do I convert it into a single string?
a = 'abcd'
Answers:
Use the join method of the empty string to join all of the strings together with the empty string in between, like so:
>>> a = ['a', 'b', 'c', 'd']
>>> ''.join(a)
'abcd'
This works in many popular languages like JavaScript and Ruby, why not in Python?
>>> ['a', 'b', 'c'].join('')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'join'
Strange enough, in Python the join method is on the str class:
# this is the Python way
"".join(['a','b','c','d'])
Why join is not a method in the list object like in JavaScript or other popular script languages? It is one example of how the Python community thinks. Since join is returning a string, it should be placed in the string class, not on the list class, so the str.join(list) method means: join the list into a new string using str as a separator (in this case str is an empty string).
Somehow I got to love this way of thinking after a while. I can complain about a lot of things in Python design, but not about its coherence.
This may be the fastest way:
>> from array import array
>> a = ['a','b','c','d']
>> array('B', map(ord,a)).tostring()
'abcd'
h = ['a','b','c','d','e','f']
g = ''
for f in h:
g = g + f
>>> g
'abcdef'
If your Python interpreter is old (1.5.2, for example, which is common on some older Linux distributions), you may not have join() available as a method on any old string object, and you will instead need to use the string module. Example:
a = ['a', 'b', 'c', 'd']
try:
b = ''.join(a)
except AttributeError:
import string
b = string.join(a, '')
The string b will be 'abcd'.
The reduce function also works
import operator
h=['a','b','c','d']
reduce(operator.add, h)
'abcd'
You could also use operator.concat() like this:
>>> from operator import concat
>>> a = ['a', 'b', 'c', 'd']
>>> reduce(concat, a)
'abcd'
If you’re using Python 3 you need to prepend:
>>> from functools import reduce
since the builtin reduce() has been removed from Python 3 and now lives in functools.reduce().
If the list contains numbers, you can use map() with join().
Eg:
>>> arr = [3, 30, 34, 5, 9]
>>> ''.join(map(str, arr))
3303459
besides str.join which is the most natural way, a possibility is to use io.StringIO and abusing writelines to write all elements in one go:
import io
a = ['a','b','c','d']
out = io.StringIO()
out.writelines(a)
print(out.getvalue())
prints:
abcd
When using this approach with a generator function or an iterable which isn’t a tuple or a list, it saves the temporary list creation that join does to allocate the right size in one go (and a list of 1-character strings is very expensive memory-wise).
If you’re low in memory and you have a lazily-evaluated object as input, this approach is the best solution.
If I have a list of chars:
a = ['a','b','c','d']
How do I convert it into a single string?
a = 'abcd'
Use the join method of the empty string to join all of the strings together with the empty string in between, like so:
>>> a = ['a', 'b', 'c', 'd']
>>> ''.join(a)
'abcd'
This works in many popular languages like JavaScript and Ruby, why not in Python?
>>> ['a', 'b', 'c'].join('')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'join'
Strange enough, in Python the join method is on the str class:
# this is the Python way
"".join(['a','b','c','d'])
Why join is not a method in the list object like in JavaScript or other popular script languages? It is one example of how the Python community thinks. Since join is returning a string, it should be placed in the string class, not on the list class, so the str.join(list) method means: join the list into a new string using str as a separator (in this case str is an empty string).
Somehow I got to love this way of thinking after a while. I can complain about a lot of things in Python design, but not about its coherence.
This may be the fastest way:
>> from array import array
>> a = ['a','b','c','d']
>> array('B', map(ord,a)).tostring()
'abcd'
h = ['a','b','c','d','e','f']
g = ''
for f in h:
g = g + f
>>> g
'abcdef'
If your Python interpreter is old (1.5.2, for example, which is common on some older Linux distributions), you may not have join() available as a method on any old string object, and you will instead need to use the string module. Example:
a = ['a', 'b', 'c', 'd']
try:
b = ''.join(a)
except AttributeError:
import string
b = string.join(a, '')
The string b will be 'abcd'.
The reduce function also works
import operator
h=['a','b','c','d']
reduce(operator.add, h)
'abcd'
You could also use operator.concat() like this:
>>> from operator import concat
>>> a = ['a', 'b', 'c', 'd']
>>> reduce(concat, a)
'abcd'
If you’re using Python 3 you need to prepend:
>>> from functools import reduce
since the builtin reduce() has been removed from Python 3 and now lives in functools.reduce().
If the list contains numbers, you can use map() with join().
Eg:
>>> arr = [3, 30, 34, 5, 9]
>>> ''.join(map(str, arr))
3303459
besides str.join which is the most natural way, a possibility is to use io.StringIO and abusing writelines to write all elements in one go:
import io
a = ['a','b','c','d']
out = io.StringIO()
out.writelines(a)
print(out.getvalue())
prints:
abcd
When using this approach with a generator function or an iterable which isn’t a tuple or a list, it saves the temporary list creation that join does to allocate the right size in one go (and a list of 1-character strings is very expensive memory-wise).
If you’re low in memory and you have a lazily-evaluated object as input, this approach is the best solution.